What is this?

People from Toronto all the way to Newcastle and Peterborough heard the same sound that was widely reported on social media across the Greater Toronto Area on Christmas Eve. It was also reported north of the city in Markham.

“We heard it again around 2:30 and 4,” said Jennifer MacKinnon of Newcastle. “Did the obligatory check to make sure it wasn’t someone falling out of bed.”

Frost quakes, or cryoseisms, occur when there is a rapid drop in temperature. Water that is in the ground freezes. Because it has nowhere else to go, it cracks the soil or rock causing a loud noise and the shaking of the ground.

These types of cryoseisms are rare and poorly understood. There are two types of cryoseisms: a frost quake and an ice quake. An ice quake occurs over bodies of water such as lakes and rivers.

The daytime high temperature in Peterborough and Ajax on Sunday was 4 C, which dropped to -17 C in the early morning hours of Monday.

“That’s definitely what it was,” said David Phillips, Senior Climatologist with Environment Canada. “It was perfect weather, the perfect condition: not a lot of snow on the ground so the sound wasn’t muffled; it was a very quick drop in temperature; the ground had been saturated from all that wet weather last week, so there was lots of water and ice in the ground.

“So when the temperatures dropped, a little bit of melting a day or two before…it was like the perfect storm of frost quakes.”

“I would say with a sudden drop in temperature that the frost quakes are a very likely explanation,” Catherine Woodgold, a seismologist with Earthquakes Canada said.

But would that explain the widespread accounts of it sounding like something had fallen on a roof?

Woodgold offered an alternative explanation: “It could be an effect with the house itself. Like the house itself could be shifting to the response in the temperature,” she said. “The house could be doing something, like when you take a cookie sheet and bend it and it makes a sudden noise.”

However, it doesn’t explain why many of the reports come around the same time across a wide area. And why is this one of the first times this has been widely reported across southern Ontario when we are no stranger to the ups and downs of temperatures?

The only thing that is different from the usual Ontario weather at this time of year is the ice storm. However, following the warmer temperatures on the weekend, very little ice remains on rooftops.

An article in the Carleton University’s monthly magazine, Carleton Now, suggests that houses can make cracking sounds in very cold weather, when there is a large variation in temperature. Fibrous materials, such as wood, can shrink rapidly, causing cracking sounds. Could this be the cause of the boom?

Whether or not it is indeed a frost quake, a lot of people are having fun with it.